This invention relates to phase-lock loops and more particularly to an improved method of locking the frequency of a master oscillator in a narrowband phase-lock loop to a pilot tone.
A phase-lock loop generally comprises an oscillator such as a voltage controlled oscillator, a phase detector, and a low pass filter. Manual adjustment of the oscillator frequency is feasible only on very stable oscillators with low aging rates. If the long term stability of the oscillator is such that aging of the frequency determiming element (i.e., a crystal) thereof places its operating frequecy outside the loop's capture range within a specified period of time, then manual adjustment is not acceptable. Although complex oscillator circuits are available that are very stable over long time intervals such as one year, they are very expensive. A sweep circuit is therefore most often used to compensate for such aging. The sweep circuit varies the oscillator frequency over a broad range when the loop loses lock and causes it to reacquire lock. Problems encountered when using a sweep circuit with a narrowband phase-lock loop are that a long time interval may be needed to reacquire lock since slow sweep rates are required, and that a momentary loss of an input or reference signal to the phase detector most often pulls the loop out of lock. These are problems which must be addressed in satellite telephone communication systems where it is necessary to lock a master oscillator to a pilot tone. In such equipment it is particularly desirable to employ a very narrowband phase-lock loop for maximum rejection of noise in the input-reference pilot signal where the output of the loop is used as a reference in satellite earth station equipment and multiplied up to a much higher frequency for transmission. In such satellite applications a natural phenomena occurs approximately twice a year that causes a conventional phase-lock loop in satellite earth station equipment to lose lock. This condition may occur because of alignment of the satellite with the sun such that the satellite receiving antenna looks directly into and receives microwave radiation from the sun. When it is possible to re-establish communication, it is desirable that the loop rapidly-automatically regain lock.
A conventional analog sweep circuit produces a ramp voltage that is combined with a correction voltage from the phase detector, with the resultant signal being applied to a control input of the master oscillator. The ramp voltage continuously varies the carrier frequency until it is aligned with the pilot frequency of the input signal. At this point the correction voltage from the phase detector overrides the sweep voltage and the loop becomes locked. Even a momentary loss of the pilot-reference signal, however, causes the phase detector's output voltage to go to zero. The ramp voltage is then predominent once again and can easily drive the carrier frequency outside the loop's lock range. When this happens the conventional loop will not reacquire lock until a full cycle of the ramp voltage has occurred. A digital sweep circuit generally comprises signal detectors and logic circuitry to determine whether loss of lock is due to the loop losing its reference or to aging of the oscillator. Such a sweep circuit sweeps by increments when the reference is present and the loop is out of lock and can remember the last value of the sweep voltage if the reference is lost only briefly. Both of these techniques have the common drawback of requiring very slow sweep rates when they are used in narrowband phase-lock loops. This is due to bandwidth limitations imposed by the loop filter. It is known that the sweep rate for phase detectors having sinusoidal outputs cannot exceed the square of the loop's natural frequency. If the sweep rate is faster than this limit, the control voltage passed through the loop filter lags behind the sweep voltage at the oscillator sufficiently that when the control voltage at the oscillator places the oscillator frequency within the loop's capture range, the control voltage from of the phase detector has actually increased sufficiently to subsequently move the oscillator frequency outside the capture range, i.e., there is a delay. Thus, when loop transients have settled out, the control voltage at the oscillator will equal the sweep voltage at the output of the phase detector and the loop will be pulled through the lock range. Very slow sweep rates, and thus very long acquisition times, are therefore normally required for narrowband phase-lock loops.
An object of this invention is the provision of an improved method of locking a master oscillator signal in a narrowband phase-lock loop to a pilot tone.